This invention pertains to turbofan engines and more particularly to the variable bypass ratio variety thereof.
Advanced technology turbofan engines are being designed to operate throughout a large spectrum of flight conditions. Unlike present day transports which operate primarily at subsonic speeds, advanced technology transports will operate both at subsonic and supersonic velocities. It has become apparent that engines representing prior art configurations are limited to efficient operation in one or the other, but not both, of those modes.
Low bypass turbofan engines have been found to operate most efficiently at supersonic velocities. On the other hand, high bypass ratio engines have been found to be most efficient at subsonic operation. Together, these facts have restricted prior art engine applications to one or the other operational mode. The ideal which has long been sought, and which is provided by the present invention, is a turbofan engine capable of operating both in high bypass and low bypass conditions so that a single engine can operate efficiently both at subsonic and supersonic speeds.
A contemporaneous problem has arisen with the new-found public concern in the noise generated by operating aircraft jet engines. There are many contributors within an individual engine to its overall noise output. With respect to high bypass fan engines, one of the chief contributors has been noise generated by rotating fan blades. With regard to low bypass engines, one of the chief contributors has been "jet noise" which is a result of extreme pressure and velocity differentials between the efflux from the engine and the atmosphere surrounding the engine and the shearing action between these fluids. Attempts at solving the jet noise problem have involved complex nozzle arrangements which have detracted from operating efficiency and desirable engine simplicity. The present invention employs a nozzle configuration which is uniquely suited to the operating parameters of an engine otherwise according to the present invention, in order to reduce jet noise without complicating the engine or reducing its efficiency.
With regard to prior attempts at utilization of variable bypass principles, the complexity of these attempts has led to their failure. For example, retractable fans, variable area turbines, and variable pitch fans and compressors have been designed in forms which have been inappropriate to efficient operation. One further attempted configuration involved a "compound" engine, which was designed to utilize a pair of engines in tandem with one or both engines functioning depending upon the mode of operation. Such a system has obvious failings--for example, the dead weight associated with the portions of the engine at times not in use.
A more recent attempt at designing a practical variable cycle engine involves selective direction of a fan stream through alternative fan ducts. A valving mechanism performs the switchover between ducts. One negative characteristic of this type variable bypass engine is the fact that, during switchover, at least a split second is involved wherein flow to both fan ducts is retarded, and thus fan stall is a distinct possibility. The present invention overcomes this disadvantage by providing for separate fan ducts and means for modulating air flows therethrough and varying the proportion of flows therebetween without the necessity of closing off either or both to flow. Continuous operation of at least one fan duct provides for utilization of the present invention without stall.